Microporous inorganic solids, such as synthetic and naturally-occurring zeolites, have found wide use in such applications as molecular sieves, filtration, and purification materials, due to their large internal surface area. However, because of the limitation of pore size of these materials to typically no smaller than about 1.5 nanometers (nm), new synthetic methods have been investigated to extend the range of available pore sizes of porous inorganic materials, such as metal oxides. Such methods include using templates of surfactant liquid (Göltner et al., Adv. Mater., 9(5):431 (1997)) and colloidal (Velev et al., Chem. Mater., 10:3597 (1998)) crystals to produce mesoporous materials with pore diameters of about 2 nm to about 10 nm. These new methods successfully expand the porosity of zeolite-type materials. However, although these methods provide useful materials for expanded opportunities in the fields of molecular sieving and chemical adsorption, these methods have the disadvantage of requiring a secondary hydrothermal process to remove the surfactant. Such thermal processes may result in collapse of the porous structure. Furthermore, most templating procedures in batch reactor operations require time-consuming methods, one-time-only use of the templating material, and auxiliary solvent extraction. Such considerations may make otherwise promising applications using the porous materials commercially uneconomical.
Nanoporous alumina is a preferred product because it is increasingly being used in adsorption and catalysis, wherein their large surface area, pore structure, and unique surface chemistry play essential roles (Kodas et al., “Alumina Powder Production by Aerosol Processes,” Alumina Chemicals: Science and Technology Handbook, The American Ceramic Society, Inc. (1990)).
Metal and metal oxides may be useful as a porous inorganic material in such applications as previously described, provided an adequate porosity is achieved. Metal and metal oxide particles may be easily produced by the spray pyrolysis process, which is one of the simplest and most industrially viable production methods in use. Spray pyrolysis involves the use of one or more precursors dissolved in a solvent and aerosolized into a droplet stream, which is then typically processed in a tubular reactor or flame (Zachariah et al., J. Mat. Res., 6:264(1991)). Typically, solvent evaporation is accompanied by precursor precipitation, and a thermally driven reaction to produce the final product. A wide variety of materials have been produced by this method using single and multi-component metals and metal-oxides (Kodas et al., Aerosol Processing of Materials, Wiley-VCH (1999)). A well known approach using spray pyrolysis to produce metal oxides involves using metal nitrate salts, which are readily available, have reasonable solubility in water, and decompose at moderate temperatures (less than about 500° C.). Conventional spray pyrolysis processes may, however, provide hollow particles of metal oxides (Dubois et al., J. Am. Ceram. Soc., 72(4):713 (1989); Gadalla et al., J. Mater. Res., 5(12):2923 (1990); Senzaki et al., J. Am. Ceram. Soc., 78(11):2973 (1995)) which may not provide sufficient particle surface area for an intended application. There is a need, therefore, for a spray pyrolysis process that provides a nanoporous metal oxide material that is able to provide adequate particle surface area for an intended application.
Recently, mesoporous silica particles have been produced by a spray pyrolysis method, in which polystyrene spheres and surfactants are employed to produce multiphased self-assembly nanostructures in an evaporating droplet mixed with tetraethylorthosilicate (TEOS, Si(OCH2CH3)4), ethanol, and water (Fan et al., J. Non-Crystalline Solids, 285:71 (2001)). However, this process has the disadvantage of requiring a secondary thermal calcination process to remove the nanophased additives from the matrix of the silica/surfactant/polystyrene spheres that, as indicated above, may cause a collapse of the particle's porous structure. Furthermore, this method requires the use of polystyrene spheres, which are an expensive precursor.
In a related work, nanoparticles have been produced using a salt-assisted spray pyrolysis method. In this approach, molten salt was used to slow down the growth rate of the particles (Xia et al., Adv. Mater., 13(20):1579(2001)). Similar experiments have been conducted in which the salt matrix is generated in-situ by the reaction of sodium with metal halides (Ehrman et al., J. Mater. Res., 14(4):1664 (1999)). Models have also been developed for coagulating nanoparticlcs, in which the nanoparticles are treated as immiscible liquid entities (nanodroplets) within much larger liquid aerosol drops composed of salt or droplets of another liquid medium (Struchtrup et al., J. Aerosol. Sci., 32:1479 (2001); Efendiev et al., Chem. Eng. Sci., 56:5763 (2001)). The growth rate and number concentration of nanoparticles in the liquid droplets are strongly dependent on the viscosity of the major phase. Since viscosity is highly temperature sensitive, temperature could be employed to change the growth rate of nanoparticles within the liquid droplets (Struchtrup et al., J. Aerosol. Sci., 32:1479 (2001); Efendiev et al., Chem. Eng. Sci., 56:5763 (2601)). The above-described salt-assisted spray pyrolysis methods, however, do not necessarily provide nanoporous materials. Thus a spray pyrolysis method that is able to provide the desired nanoporous materials is needed.